Using light to describe the ancient world

Archive for July, 2011

Hyperspectral near infrared mapping of fossil bone

Pleistocene horn cores from bovids (like the springbok shown here) were recently analysed using hyperspectral near infrared imaging. We were able to make spectral maps of the horn cores, and determine that the bones had been exposed to a substantial groundwater flow. Might this be bad news for isotope studies of these fossils? We shall have to find out.

Spectra can be richly informative, as I have hopefully shown in earlier entries. Take any fossil, position it under the spectroscopic instrument of your choice, and you will likely be rewarded with some otherwise invisible information. X-ray diffraction will reveal mineralogy, x-ray fluorescence will give chemistry, mid-infrared and Raman will give ionic composition. Most of the time, the information couldn’t be gleaned from that spot on the fossil using any other method. And most of the time, spectroscopic techniques reveal information about a single spot per analysis.

In the surge of every advancing technology, spectroscopic instrumentation is moving beyond single spot analyses and entering an era where entire surfaces are rapidly mapped. An example of this is hyperspectral near infrared imaging. We have recently published results from a SisuChema imaging system, administrated by Professor Alvaro Viljoen of the Tshwane University of Technology in Pretoria, South Africa. Using this instrument, we were able to collect near infrared spectra from the surfaces of Pleistocene bovid horn cores. Thousands of spectra. The instrument collected roughly 1000 spectra every square millimetre, across a 10 mm wide transect. By taking multiple transects we were able to prepare detailed maps of the horn cores that showed exactly where certain near infrared wavelengths were absorbed. Why was that important? The hyperspectral NIR maps revealed the suffusion of ancient groundwater.

A NIR spectrum of fossil bone is generally uninformative. Near infrared is only really useful for materials made from the lightest elements, which fortuitously includes carbonates and clays. The hyperspectral maps revealed that secondary minerals had been deposited deep inside the tiniest pores and cracks in the bovid horn cores, meaning that a substantial amount of groundwater had flowed into and through the bones. Groundwater is the agent of diagenesis, which means that these fossil bones may no longer carry vital information. The fossils we chose to study are from a suite of sites where geochemical signals have been used to understand the ancient environment. If anything, our data show that these sites may not be giving trustworthy answers. Our next step is to study the isotopic compositions and histology of these bones, to determine whether groundwater has stripped away any analytically useful signals.

So, hyperspectral NIR mapping of Pleistocene fossil bones is a great way to assess whether they have been diagenetically altered.

Thomas DB, McGoverin CM, Chinsamy A, Manley M. 2011. Near infrared analysis of fossil bone from the Western Cape of South Africa. Journal of Near Infrared Spectroscopy 19:151-159.


Raman spectroscopy of fossil teeth

The width and position of a Raman spectral band has been correlated against alteration in fossil bone.

Chemistry has become the foundation for many paleontological explorations. Very recently, chemical signals in dinosaur teeth revealed that long-necked sauropods were warm to the touch. This pinnacle example of paleo-chemistry is one of the many lofty studies that have helped understand life on ancient Earth. Biological conclusions are occasionally based on altered fossils, however, whose chemical signals give seductive, but spurious, results. Much effort has been devoted to identifying altered bones before they are analysed. So far, no single technique can confidently tell us whether the chemical signals in fossil bones are pristine, but new techniques are still being developed.

One of these new techniques uses Raman spectroscopy. Based on the scattering of light, Raman spectroscopy is a non-destructive method for analysing chemical compositions. Take fossil bone for instance – a Raman spectrum reveals the chemical composition of the bone mineral lattice. Using Raman spectroscopy alone, we can uncover the chemical differences between modern and fossil bone, and the differences between altered and unaltered fossil bone. I recently coauthored a study that applied Raman spectroscopy to fossil teeth – check out the abstract below….


Thomas DB, McGoverin CM, Fordyce RE, Frew RD, Gordon KC. 2011. Raman spectroscopy of fossil bioapatite — A proxy for diagenetic alteration of the oxygen isotope composition. Palaeogeography, Palaeoclimatology, Palaeoecology. doi:10.1016/j.palaeo.2011.06.016

Fossil bioapatite may yield biogeochemical signals of paleoenvironments captured by living organisms. Bioapatite may be diagenetically altered, however, with ions added or removed post-mortem; such change is typically assessed using destructive and demanding techniques. Here, Raman spectroscopy is used as a rapid and non-destructive way to identify significant diagenetic alteration of fossil bioapatite. We found spectral parameters of phosphate symmetric stretching (ν1-PO43−) to be very sensitive to variations in apatite chemistry, particularly with respect to common diagenetic components (CO32−, F, Sr2+). The Raman spectral parameters were subsequently applied to a set of modern (biogenic) and geologic (magmatic) apatite samples as potential endmembers for diagenetic alteration. Raman spectra were also collected from enamel and dentin (respectively resistant vs. alteration-prone) of fossil teeth. Phosphate-oxygen isotopic values from the same enamel–dentin samples were used as an index of alteration and provided definition of Raman spectral parameters as relates to diagenetic alteration. Diagenetically altered samples were characterised by spectra with ν1-PO43− widths (at half maximum height) less than 13.0 cm− 1, and ν1-PO43− band positions greater than 964.7 cm− 1. Raman spectroscopy is shown to have potential as a tool for pre-screening fossil apatite samples before further analyses.


The List

I have just started a new feature – The List. On this page, linked right at the top, will be an evolving list of all the papers I encounter that have applied spectroscopy to fossils. This will be a reference guide, and the means for tracing trends and developments.

#News: Fossil feather colouration

Roy Wogelius and colleagues describe feather pigmentation using synchrotron sourced x-ray fluorescence.

Two papers were published early last year that described colour in fossil feathers. One in Nature, and one in Science. These studies were based on a brilliant deduction by Jacob Vinther, who realised that colour structures seen in modern feathers may be preserved in fossils. This work was later applied to a newly described fossil penguin, with curious results. The colour structures identified by Vinther are associated with trace elements, which can also be used to reconstruct colour patterning in fossil feathers. Roy Wogelius and colleagues recently mapped the surface of a Late Cretaceous bird fossil, using x-ray fluorescence, allowing them to reconstruct provide even more insight into fossil colour patterns. The Royal Society of Chemistry have provided an excellent summary of the article.

Wogelius RA, Manning PL, Barden HE, Edwards NP, Webb SM, Sellers WI, Taylor KG, Larson PL, Dodson P, You H, Da-qing L and Bergmann U. 2011. Trace Metals as Biomarkers for Eumelanin Pigment in the Fossil Record. Science DOI: 10.1126/science.1205748

Image from Wikimedia Commons

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